The present invention relates to a device for holding screws securely in contact with a driving bit during positioning and driving of the screw. The device is mounted upon a screw driver bit, particularly a bit intended for use in a motor powered driver and more particularly, for surgical procedures involving driving screws into bone.
The device is capable of undergoing repeated steam sterilization without impairment of function, may be quickly disassembled without the need for tools for rapid change of screw driver bit size or style, and is capable of accommodating screws of a variety of sizes and head shapes. The device provides positive, locking screw holding engagement yet permits angular screw displacement of about 10.degree. from axial direction. The device contributes little toward operator fatigue because of its compact size, light weight and use of counter-balanced spring bias forces.
Devices previously used for surgical procedures, primarily bone fixation, although of similar size, operated on a totally different principle. These devices utilized a plurality of spring-like gripping elements each having a forward groove for gripping the screw head. They usually had no, or poor, screw locking means, were prone to malfunction, were subject to frequent premature release of the screw, and provided for little or no angular displacement of the screw from axial alignment so that a screw which was misdirected into a pilot hole could cause cracking or splintering of the bone.
Screw holding devices intended for use by craftsman or in the trades were much more dependable than those used by the surgical profession but were bulky, clumsy and complicated and hence, poorly qualified for adaptation to this highly specialized use. Also, for devices used by a craftsman, equipment cost is a primary concern with little worry directed to possible damage to the screw threads and heads. The damage to screw threads and screw heads caused by these devices is of slight importance in, e.g., sheet metal work but would be of extreme concern in a delicate surgical procedure.
Many of the craft directed prior art devices utilized ball members as well as concentric sleeves, springs, etc. for their gripping action.
Thus, Schmitt U.S. Pat. 2,840,126, although seemingly incorporating many features which are roughly comparable to those of the present invention, differs markedly. Schmitt uses the bit to back up the screw while the balls are forced backward along the screw with possible damage to the screw threads in the process. Further, the device is necessarily fastened onto the motor rather than to the bit because of its large size and weight due partly to the unique screw loading mechanism. Such a mechanism would not only render the device exceedingly clumsy for use in surgical procedures but would also be difficult to clean, disassemble, and sterilize. Schmitt's device appears to have been primarily designed for attachment to a stationary screw-press machine. The device could easily cause scoring or marring of screw heads during engagement, an unacceptable condition for surgical usage. The device also causes positive locking of the screw when loaded, a feature which does not allow any angular movement of the screw to permit compensation for an angularly displaced pilot hole without stressing the substrate. Another disadvantage is that Schmitt's device can only accommodate a given length of screw unless the bit is changed, which necessitates an extensive operation involving the use of tools. Finally, it is doubted whether the device could be used to extract a screw since a screw cannot be engaged by the bit from the front.
Luber, U.S. Pat. No. 2,845,968, is an even more primitive device in that the screw is loosely held by forcing the device forward against the substrate whereby the end of the device, containing balls backed by resilient material, is forced back relative to the bit and screw causing the balls to center the screw. This device would be entirely impractical for exacting surgical work since at no point can the screw be seen for exact positioning. This device would cause damage to both the head and threads of the screw, would be clumsy to use and could hardly be used to extract a screw. The device also would require the use of excessive pressure against a patient causing fatigue to the surgeon.
Taylor, U.S. Pat. No. 3,181,580, is one variation of the many available spring-clip type screw holders. Among the main disadvantages of spring-clip holders are:
(1) They allow for no angular misalignment of the screw from the axial direction.
(2) They are prone to accidental premature release of the screw.
(3) They are usually intolerant of slight screw size changes.
(4) They are prone to fatigue of one or more of the spring elements, a malfunction which is difficult to repair.
(5) They do not have a positive locking mechanism.
McKenzie, U.S. Pat. No. 3,244,208, is another variation of the spring-clip screw holder. This type suffers the further disadvantage that while it can hold flat-head screws such as the sheet metal screws for which it was designed, it has very poor holding power for a screw such as a surgical screw which is designed to fit into a countersunk hole.
Morifuji, U.S. Pat. No. 3,298,410, discloses a simple low cost device for use with a manually operated screwdriver. It does not appear that the resilient holding means can provide the secure locking required for surgical usage with a powered driver.
Eby, U.S. Pat. No. 3,901,298, another spring-clip holder with the disadvantages previously listed, also has the further disadvantage of requiring manual release of the screw as contrasted with the automatic release of the present invention.
Belgium Pat. No. 500,711 relates to a device which requires too much manual dexterity for use in surgical procedures.
Lesner, U.S. Pat. No. 3,967,664, suffers from the disadvantage, common with many of the other prior art patents, that the operator must exert considerable pressure upon the device while driving screws to overcome the spring bias built into the device. For example, the first embodiment described by Lesner requires a driving force counter to the combined action of three heavy springs. This not only causes undue fatigue to the operator but in the case of surgical procedures, would cause entirely unacceptable stress and strain upon the surgical repair site. By contrast, the device of the present invention uses counteracting springs to lessen the overall bias. During the majority of the screw travel there is no spring bias to overcome. During the final screw tightening, there is a very low spring reaction which is almost unnoticeable. Lesner's balls are restricted from penetrating more than a minor amount inwardly toward the screw, restricting gripping power, angular displacement of the screw and the ability of the device to handle screws of various head sizes and shapes. Lesner's device attaches to the driver housing, restricting interchangeability, cleaning and sterilization. The device undergoes very sensitive transition changes within the locking mechanism between loading and driving which would appear to inevitably result in malfunction, particularly if the springs are not proportioned correctly or if the screw is started into an angularly misaligned pilot hole. In the second embodiment, the screw is entirely unlocked from gripping contact during the entire driving operation.